Device for sampling aerosols



Sept. 15, 1970 C. LASSEUR ET AL DEVICE FOR SAMPLING AEROSOLS Filed Jan.22, 1968 INVENTORS 64/14/25 LA 556L111 L'LA #JE' TOA/ /eL/E/QJ'EAM'CLAUJE ZERB (S MfW' ATTORNEYS United States Patent 3,528,279DEVICE FOR SAMPLING AEROSOLS Claude Lasseur, Clamart, Claude Tonnelier,Verneuilsur-Seine, and Jean Claude Zerbib, Antony, France,

assignors to Commissariat a lEnergie Atomique, Paris,

France Filed Jan. 22, 1968, Ser. No. 699,453 Claims priority,application France, Jan. 31, 1967, 93 277 Int. Cl. B01d 25/04,- G011!1/24 U.S. C]. 73-28 6 Claims ABSTRACT OF THE DISCLOSURE A device forsampling aerosols contained in a fluid, comprising means for circulatingsaid fluid through a filter for retaining aerosols. Said filterseparates a sampling chamber from a chamber for the discharge of saidfluid, said discharge chamber being disposed within said samplingchamber and so arranged as to define therewith an annular space throughwhich the fluid is circulated upstream of the filter. Radial fins fordistributing said fluid are provided in said annular space.

tive of the aerosols contained in the fluid while permitting analysis ofthe deposit during the sampling operation.

It is already known to make use of sampling devices for the main purposeof analyzing radioactive aerosols contained in the atmosphere, in whicha radiation detector is placed against the filter for the purpose ofproviding a continuous measurement of the intensity of radiation emittedby the aerosols retained on the filter. In devices of this type, thedetector is located inside the sampling chamber; it must be placed asnear as possible to the filter and cover the entire surface of thislatter. Consequently, the pipe through which the fluid is admitted intothe sampling chamber must be placed laterally with respect to thefilter.

These known devices thus have a disadvantage in that they provide deadzones for the circulation of the fluid in which a proportion of theaerosols is consequently deposited before these latter reach the filter.In addition, the deposits which are formed on the filter are nothomogeneous and this absence of homogeneity has unfavorableconsequences, particularly in regard to the accuracy of radiationmeasurements which indicate the concentration of radioactive aerosols.In fact, the detection probes (such as a scintillation counter, forexample) are usually calibrated for a homogeneous deposit. In addition,such probes are more sensitive to the radiations emanating from thecenter of the filter than to the radiations which emanate from theperiphery.

One of the aims of the present invention is therefore to ensurehomogeneity of the aerosol deposit on the filter in order to derive themaximum benefit from the higher efficiency of the detection means in thecentral zone and also in order to make it possible after removal of the3,528,279 Patented Sept. 15 1970 "ice filter from the device to takesubsequent laboratory measurements by means of calibrated probes.

A further aim of this invention is to reduce as far as possible theabrupt loss of fluid velocity in the sampling device and to prevent anypart of the aerosols from being deposited by impact within the samplingchamber before reaching the filter, and thus affecting the measurements.It has therefore been sought to avoid as far as possible the presence ofany dead zones in which the aerosol deposits are liable to form upstreamof the filter and to restore the velocity of any particle which hassustained a first impact against the walls of the sampling chamber.

The device for sampling aerosols in accordance with the invention makesit possible to satisfy the essential conditions set forth above moreeffectively than any of the comparable devices of the prior art.

The device is essentially characterized in that the discharge chamber isdisposed inside the sampling chamber and defines with this latter anannular space through which the fluid is circulated upstream of thefilter and in which are disposed radial fins for the distribution ofsaid fluid.

According to a preferred embodiment of the invention, the samplingchamber has a progressively increasing cross-section from a fluid-inletpipe up to said fins, and the discharge chamber is defined by a cone,the base of which is occupied by a filter and the apex of which opensinto a discharge pipe which passes through the side wall of the samplingchamber.

The whole device is advantageously endowed with symmetry of revolution.

In the device according to the invention, the presence of fins in theannular space which is defined by the chambers for sampling anddischarge of fluid ensures a radial distribution of the fluid: eachfluid stream is oriented towards the center of the filter and parallelto this latter, the space available between the filter and the wall ofthe sampling chamber being preferably small. In consequence, a notnegligible fraction of the fluid flow reaches the center of the filterand each portion of said filter, whether said portion is located in thecentral zone or on the contrary in the peripheral zone, is traversed bya part of the fluid and retains the aerosols contained therein. Thedeposit obtained is thus uniformly distributed over the entire surfaceof the filter.

It has also been found that the quality of the deposit obtained and thereproducibility of measurements were closely related to the flow regimeof the fluid under analysis within the device whereas, up to the presenttime, the influence of the flow conditions had been whollyunappreciated.

According to a secondary feature of the invention, the device is sodimensioned that the state of flow of the fluid is laminar at the levelof the fins and of the filter, this being naturally the case in respectof a predetermined range of flow rates. On the other hand, the fluidadvantageously flows in a turbulent state within the pipe through whichit is admitted into the sampling device in order to prevent thedeposition of any part of the aerosols upstream of the device.

Within the sampling chamber, the change from turbulent flow to laminarflow preferably takes place immediately upstream of the radial fins.Inasmuch as said fins tend to reduce the Reynolds number, a free laminarflow regime is thus established at the level of the filter. Furthermore,the instability zone corresponding to the change of flow regime isaccordingly reduced.

Said laminar flow combined with the particular shape which is given tothe sampling chamber has the effect of carrying up to the filter itselfall the particles con- 3 tained in the fluid which penetrates into thechamber. This effectively circumvents one essential defect of prior artdevices in which aerosol deposits upstream of the filter impaired thevalidity of the sampling and additional- 1y gave rise to contaminationof the sampling chamber.

Referring to the single figure of the accompanying drawings, there willnow be described one particular embodiment of the sampling device inaccordance with the invention as chosen by way of example and not in anysense by way of limitation.

In the sampling device which is shown diagrammatically in the figure,the body 1 is removably fixed inside a cylindrical support tube 2. Saidtube performs the function of a support, not only for the removablesampling head, but also for a scintillation counting unit 3 formeasuring the radiation emitted by the aerosol deposit as well as forlead rings which are disposed around the tube 2 for the purpose ofshielding the unit from stray radiations. Said rings are not shown inthe figure. The counting unit 3 for measuring the radiation is showndiagrammatically in the figure.

A mounting ring 4 is screwed into the support tube 2. The counting unit3 is intended to be fitted over said ring whilst the sampling head issuspended from the lower end thereof. The dot-dash line on the uppersurfaces of element 4 schematically illustrates the limit of thecounting unit which allows for the examination of the radiation that isemitted by the deposit of the aerosol.

To this end, the body 1 which is manipulated by means of a handle 6 hasan extension in the form of a sealing collar 7 which is fixed on thering 4 by means of a bayonet system. A seal 8 ensures imperviousness togases and to light.

The body 1 defines with the collar 7, the ring 4 and the counting unit aleak-tight sampling chamber 10. The fluid to be analyzed, which isusually a gas and more especially atmospheric air, is admitted axiallyinto said sampling chamber via an inlet pipe 12 which is fitted with aquick union and can thus be rapidly connected to the body 1. Said bodyhas a conical shape and accordingly widens from the inlet pipe 12 to thesealing collar 7.

Inside the chamber 10, there is located a discharge chamber 13 which isdefined by a suction cone 14, the large base of which is closed by afilter 15. Said filter consists of ordinary filter paper and issupported by a metallic grid 16. The assembly of grid and filter ismaintained by means of a locking ring 17.

The large base of the suction cone 15 as shown in the figure is upwardlyoriented in the same manner as the body 1. The distance between saidlarge base and the top end of the chamber is small, and essentiallydependent upon the conditions governing the detection of aerosolsdeposited on the filter by means of the counting unit 3. At the lowerend thereof, the suction cone 14 is connected to a pipe 18 for thedischarge of fluid which forms an elbow 20 and passes out of the chamber10 through the side wall of the body 1. Said pipe is fitted with a quickunion 21 whereby the device can be connected to an apparatus whichserves to suck the fluid from the sampling chamber through the filter15.

The suction cone 14 defines with the walls of the chamber 10(essentially formed by the body 1 which terminates in the collar 7) anannular space 22 through which the fluid must pass before reaching thefilter 15. Provision is made within said annular space for fins 24 whichare welded to the outer surface of the cone 14. Said fins are orientedradially with respect to the cone. The number of fins is chosen as afunction of the operating conditions envisaged in order to ensure that afree laminar flow is obtained upstream of the filter.

The complete assembly of sampling chamber, discharge chamber and finspossesses symmetry of revolution.

The device as a whole is dimensioned as a function Cir of the nature ofthe fluid and the rate of sampling which is contemplated in order toensure that the flow, which is in the turbulent state within the pipe12, should become substantially laminar at the level of the lower endsof the fins 24. The fins subsequently ensure a homogeneous distributionof the streamlines of fluid and tranquilize the streams of fluid byreducing the Reynolds number and preventing the inception of a helicalflow motion around the suction cone 14.

It is additionally apparent that the shape of the device is such thatthere does not exist any dead zone in which a deposit of aerosolparticles is liable to form and that those particles which have impingedon the walls of the chamber 10 and essentially on the elbow of theoutlet pipe are automatically restored to their initial velocity. Thisrestoration of velocity is ensured irrespective of the position of thedevice in space.

The device hereinabove described is also designed so that it can bereadily disassembled. A rotation of the body 1 through a few degressreleases the sampling head. The filter itself is removed after unlockingthe ring 17. This case of disassembly permits the possibility ofchecking any contamination which may be present and of changing thefilter. During normal operation, the quantity of aerosols deposited iscontinuously measured by the scintillation counting unit. The rapidwithdrawal of the movable portion from the support tube makes itpossible to replace said portion in order to perform a routine testwithout interrupting the measurements.

There now follows an example of construction which serves to explain themethod of dimensioning of the above-described device by applying knowncorrelations between the characteristics of geometry and the parametersof fluid flow.

It is known that the conditions of flow of a fluid are essentiallygoverned by the value of the Reynolds number. This number is expressedas a function of the flow velocity V, the hydraulic diameter D, thespecific density p and dynamic viscosity ,u of the fluid by thefollowing relation:

The hydraulic diameter is in turn expressed as a function of the flowcross-sectional area S and the perimeter P which is wetted by the fluid:

In the present example, the fluid being analyzed is air under normalconditions of temperature and pressure.

We therefore have:

p: l .205 10 g./cm.

and

,u=18 x 10- g./cm.s.

So far as the sampling rate is concerned, the rate of human respirationis usually taken into consideration, namely approximately 1.2 mfi/h. oncontinuous rating, or 2.4 m. /h. on a pulsed rating (time of inhalationequal to the exhalation time). However this value of flow rate is toolow to provide with good statistical accuracy a population of aerosolswhich is representative of the sampling location both from a qualitativeand quantitative standpoint. Thus, in the case of some radioactiveaerosols, a single particle per cubic meter of air can represent a highlevel of activity and a concentration which is in excess of the maximumpermissible concentration. In order to ensure a good probability ofcollection of aerosols, there has accordingly been chosen in thisexample a flow rate which is seven times higher than the standard humanbreathing rate, namely Q=8.5 m. /h.

The diameter of the filter is selected as a function of the requisiteflow rate, of the dimensions and of the concentration of the aerosolparticles encountered. A diameter which is too small would in factresult in rapid clogging of the filter and a modification of thecharacteristics of the air flow. Furthermore, the filter must besufiiciently close to the detector of the counting unit to prevent anysubstantial absorption of radiations in the intermediate layer of air.

In the particular case herein described, use is made of a filter paperwhich has a high stopping power and a diameter of 110 mm. and which isplaced at a distance of 3 mm. from the detector. A smaller distancewould not introduce any appreciable modification in the count rate butwould, on the other hand, increase the pressure losses.

The inlet pipe for the admission of air into the sampling chamber has aninternal diameter of 30 mm. In the case of the air flow rate Q underconsideration, the flow velocity is V=333 cm./ sec. A calculation of thecorresponding Reynolds number gives: Re=6700. In the case of theconventional pipes employed, this value corresponds to a turbulent airflow.

Within the annular space around the suction cone, the Reynolds number iscalculated as a function of the internal diameter D of the chamber andof the external diameter d of the cone at the level of its lower base(air outlet pipe) and of the number n of fins. We find:

In the example herein described, we have selected: D=l2.8 cms.; d=3.8cms.; "=6 fins.

For the same air flow Q, we therefore find:

This value corresponds to a free laminar flow regime.

Similarly, at the level of the filter, in the case of a diameter of 12cms. and a height of 2 mm. (taking into account the locking ring of thefilter) we find Re=840. The flow is again in a free laminar state.

The values obtained in respect of the Reynolds number show that the fiowcan be increased from 8.5 m. /h. to a rate of 15 or 20 mfi/h. whileremaining in the laminar state.

What we claim is:

1. A device for sampling aerosols contained in a pipe, comprising meansfor circulating said fluid through an aerosol filter, a conduit forsampling said fluid upstream of the filter and comprising a firstportion having a conical cross-section whose apex opens into a pipe forthe admission of said fluid and a second portion having a cylindricalcross-section which forms an extension of said first portion andcontains said filter, a fluid discharge chamber disposed within saidsecond portion of the sampling conduit and having the shape of a conewhose base is occupied by the filter and whose apex opens into adischarge pipe which transverses the side wall of said first conicalportion of the sampling conduit, and radial fins disposed within theannular space which is defined by said discharge chamber and said secondcylindrical portion of the sampling conduit, said fins being intended toensure upstream of the filter a homogeneous distribution of the fluidand a laminar flow.

2. An apparatus for sampling aerosols contained in a fluid whichcomprises a sampling conduit provided with a fluid-inlet pipe, saidsampling conduit having a progressively increasing cross-sectional areaextending from said fluid-inlet pipe, a cone-shaped discharge disposedin said sampling conduit, the base of said discharge chamber beingprovided with a filter means which separates the sampling conduit fromthe discharge chamber and the apex of said discharge chamber containinga discharge pipe which passes through the side wall of the samplingconduit, the walls of the sampling conduit and the walls of thedischarge chamber defining an annular space therebetween through whichthe fluid is introduced from the fluid-inlet pipe, and fin meansradially disposed in said annular space to provide laminar flow upstreamof thefilter means.

3. The apparatus of claim 2, wherein means is provided for detectingradiations emitted by the aerosol retained on the filter means, saidmeans being disposed externally of the sampling conduit and in closepromixity to the filter means.

4. The apparatus of claim 2, wherein the fin means are attached to thewall of the discharge chamber.

5. The apparatus of claim 4, wherein the cross-section of the samplingconduit increases from the fluid-inlet pipe up to the fin means.

6. The apparatus of claim 2, mounted in a cylindrical support tube.

References Cited UNITED STATES PATENTS 2,790,253 4/1957 Ayer 7328 X3,011,336 12/1961 Weiss 7329 FOREIGN PATENTS 828,317 1/1952 Germany.

RICHARD C. QUEISSER, Primary Examiner C. E. SNEE III, Assistant ExaminerUS. Cl. X.R. -270

